The present invention relates to a method and an electrical circuit for determining the momentary (instantaneous) frequency of a signal which is variable over time within a given frequency bandwidth.
Such a method is disclosed, for example, in "Digital Instantaneous Frequency Measurement for EW Receivers," Ralph Bauman, Microwave Journal, Volume 28, No. 2 (February, 1985) pages 147-154. For the rapid detection of high and very high frequencies (f&gt;1 GHz), essentially only special parallel methods can be employed, particularly those operating according to the so-called "interferometer principle" which originates from the optical art and uses coherent superposition of phase-shifted wave trains of the same frequency and the same amplitude. These methods basically employ one or several delay lines or delay elements to produce the necessary frequency proportional phase shifts .THETA.(f) of the input signal s(t). The mathematical relationship between the signal delay time .tau. and the signal phase shift .THETA. in a homogeneous, low-loss delay line D.sub.i is given by .THETA..sub.i (f)=2.pi.f.tau. where .tau.=1/.DELTA.f (.DELTA.f=detection bandwidth) and thus the complex output amplitude EQU B.sub.i =A.sub.i .multidot.e.sup.-j.THETA.i
applies for a complex input amplitude A.sub.i.
Frequency independent (constant) signal phases A.sub.i, formed from input signal s(t) and selected according to a certain mathematical rule, must be fed into the parallel arranged delay lines. These signal phases A.sub.i can be selected by means of a microwave stripline network or by a hybrid coupler network.
In the known method, the actual frequency detections are obtained from the parallel, delayed signal phases B.sub.i by means of frequency discriminator networks and correlators which are purely analog circuits. For this purpose, further passive phase shifting networks are employed in conjunction with high frequency mixers which, by special phase summations of the delayed complex amplitudes B.sub.i and subsequent signal mixing (quadrature mixing), form the frequency value components W.sub.i. These frequency value components W.sub.i must now be separated by lowpass filtering from the further mixing products (harmonics of s(t)) and must be combined by subsequent difference formations, e.g. by means of video differential amplifiers, so that the individual, desired analog frequency values W(f) result. Only then can the frequency values W(f) be digitalized in the lowpass filter range by means of special analog/digital converters and phase/digital converters.
Thus, in the known method, in spite of its parallel basic structure, the individual signals must travel very long processing paths through the many different components to reach the frequency value output. This, and primarily the finite bandwidth (B.sub.v &lt;100 MHz) of the differential amplifiers, requires a correspondingly long response time taq in the frequency detectors. In the conventional methods, this response time lies at taq&gt;70 ns and is thus too long for a precise evaluation of microwave pulses which have a duration of tp&lt;100 ns.
However, the demand for very fast digital momentary frequency detectors is presently growing at a fast rate for real time signal processing in satellite radio, satellite television, space travel communications, directional radio, radar and for digital UHF receivers in all commercial and military applications. This trend is decisively supported by the increased occurrence of novel, monolithically integrated microwave circuits (MMIC's), particularly digital circuits (gigabit logic) in GaAs-FET (field effect transistors) and high speed ECL (emitter coupled logic) technology.